Sunday, March 9, 2014

The Mickey Mouse Telescope

From the summit of Mount Hopkins, Arizona (December 14, 2005)

This is not another post about nocturnal mice visiting graduate student astronomers. This is about the naming of a telescope. The MMT telescope, to be precise, the large 6.5 meter mirror on top of Mount Hopkins, in southern Arizona.

The story of the MMT telescope starts many years ago. Since 1610, when Galileo Galilei first pointed a telescope to the sky, astronomers' needs have fueled an arm's race to build larger and larger telescopes. Big telescopes are doubly advantageous: they collect more light, allowing to see fainter and fainter objects, and have a larger magnification rate, making possible to see smaller details on larger distances. While Galileo's instrument had lenses, Isaac Newton was the first to realize that by using mirrors it was possible to fit the same telescope's power into smaller packages: multiple mirrors can be arranged to fold light more efficiently that a pairs of lenses. Furthermore, mirrors are cheaper to manufacture, and can be shaped and polished in much larger sizes than lenses. The invention of the newtonian telescope made possible the arm's race in modern astronomical instrumentation, culminating in the construction of the gigantic 200-inch (5.1 meter) "Hale" telescope at Mount Palomar.

The big telescope at Palomar is a monster. Completed in 1948, it was the largest until 1976, when the Russian BTA-6 telescope with its 6.1 meter diameter mirror was completed. The Hale telescope is housed in a huge dome, 42 meters in diameter and 41 meters tall, which is about the same size as the dome of the Pantheon in Rome. The 200-inch mirror, 26 inch thick and weighting over 20 tons, was made in Pyrex (the kind of glass used for heat resistant cooking containers) by Corning, the same company making the iPhone "Gorilla glass". This was the largest mirror that could be effectively built and polished with the technology of the time: its Russian successor, while almost 1 meter larger in diameter, was plagued by imperfections, and never matched the effective performances of the large Hale telescope mirror. The problem with such large mirrors is that they need to be very thick not to break under their own weight, and the thicker they are, the heavier they get. All that weight requires an enormous structure to support it. The whole things needs then to be moved around, as the telescope points to different spots in the sky: as the telescope moves, the weight shifts, deforming the shape of the mirror and producing distorted images as it falls out of alignment.

The MMT dome

This was the state of business in the late 1960's, when astronomers at the Smithsonian Astrophysical Observatory in Cambridge, MA and at the University of Arizona in Tucson started to make plans for a new large telescope to be built at Mount Hopkins. They knew that casting a large mirror was not feasible, so they thought that they could build a larger telescope by assembling 6 smaller (1.8 meter diameter each) mirrors together on the same mount, and then combine their light together as if they were part of a single telescope with an effective area of a 4.5 meter diameter mirror. Thus in 1979 the Multiple Mirror Telescope (MMT) was born, at the time the third larger aperture telescope in the world, a technological testbed for the giant mirror telescopes of the future. Combining multiple mirrors was not the only design innovation attempted for the MMT: while traditional telescopes require huge domes so that the telescope can move to track the stars, the MMT fits snugly into its square dome. The MMT can only move up and down, and it is the entire building that rotates to enable tracking. This allowed to significantly reduce the construction costs of the project, and gave the telescope the same appearance of one of these model boats built inside a bottle.

The new MMT

The MMT was in service for almost 20 years, during which it was at the forefront of technological experimentation. Its success however, was also the cause of its downfall. By the 1990s astronomers became so proficient in phasing together many small mirrors, that it became feasible to build gigantic segmented mirrors made by many smaller hexagonal pieces. In 1993 the first of the twin Keck telescopes (36 segments to form a 10 meter diameter mirror) started doing science observations from the top of Mauna Kea in Hawaii. It was time for the MMT to be updated, but what to do? The solution came from the master telescope maker at the University of Arizona, Roger Angel. He decided to resurrect the single mirror design but, rather than trying to make the telescope stiffer, he proposed to embrace its floppiness to make a thin and light mirror, that could be easily deformed. A computerized system would then continuously measure the shape of the mirror, calculate in real time the distortions due to its shifting weights, and then restore the ideal optical shape by means of hydraulic actuators pushing the mirror from below. Such intelligent mirrors, called "active optics" are now standard in the construction of modern gigantic telescopes, whose mirror are often cast in the "mirror lab" located below University of Arizona football stadium. The cavernous space hosts the big spinning furnace where the special ultralight glass is fused, and then slowly cooled. Very slowly: it takes one year for each new mirror to cool without cracking. In this lab the new 6.5 meter diameter mirror for the refurbished MMT telescope was born, and the telescope was re-dedicated in the year 2000.

And this is where the famous cartoon mouse enters in this story. As the re-dedication ceremony was approaching, the officials at the Smithsonian and University of Arizona opened a contest to find a new name for the telescope. The rule was that the new name should still initial as M, M and T to preserve the same acronym. The most obvious choice was the "Mono Mirror Telescope", which however was considered silly because most telescopes, at the time, were still made with a single primary mirror. So it was finally decided to call it the "new MMT" telescope, where MMT stands for, recursively, MMT, the name that the pioneering telescope had in its previous incarnation. Among the proposed names that didn't make it, however, there is my favorite: the Mickey Mouse Telescope, an irreverent idea that was rapidly abandoned after somebody started to worry about Disney and its layers.